Due to the ever-increasing number of connections (pincount) of integrated circuits, the use of ball grid array packages to connect integrated circuits to printed circuit boards is increasing. This facilitates the redistribution of a very fine pitch of flip-chip bump array of the integrated circuit to a much larger pitch ball grid array for attachment to the printed circuit board (PCB).
The carrier is often referred to as an interposer and can be fabricated from different materials such as ceramic, or a plastics material such as bismaleimide triazine (BT).
The carrier also functions as a heat sink by removing thermal energy from the integrated circuit by thermal conduction. Accordingly, the carrier is subjected to thermal strains.
In addition, an electronic package assembly comprising the integrated circuit, the carrier and the PCB has a number of different materials with different mechanical properties. Complex thermal stresses can occur inside the package during operation due to non-uniform temperature distributions, geometry, material construction and thermal expansion mismatches.
Typically, these days the integrated circuit is electrically connected to the carrier by a ball grid array of gold or solder bumps. Similarly, the carrier is electrically connected to the PCB by a further, larger ball grid array of solder balls. The thermo-mechanical stresses are typically severest at the solder ball interfaces between the PCB and the carrier. This can result in shearing of the solder ball connection. The problem is amplified by an increase in edge length of the carrier because of an increase in the thermal strain differences between the PCB and the carrier. An increase in edge length of the carrier is typically associated with an increase in the number of integrated circuit connections and solder balls.
Current ball grid array design is, presently, at the limit of reliability for typical integrated circuit pin counts.
Typically, a solder ball has a peak elastic shear strain value of around 0.08%. Computational experiments done by the applicant using a 500 micron thick solid Silicon carrier, 500 micron diameter solder balls at 1 millimeter pitch, a 700 micron thick PCB and a 16 millimeter side silicon chip indicated a peak shear strain value of 1.476% in the outermost ball of the package which is far above the plastic yield value of the solder ball.
This result is to be expected as the balls at the outermost edge of the package experience the greatest amount of translational shear.
As indicated in the publication of the Assembly and Packaging Section of the International Technology Road Map for Semiconductors, —1999 Edition, the most recent edition available at the time of filing the present application, in Table 59a at page 217, a pin count of a high performance integrated circuit has of the order of 1800 pins. The technology requirements in the near term, i.e. until the year 2005 indicate that, for high performance integrated circuits, a pin count exceeding 3,000 will be required for which, as the table indicates, there is, to date, no known solution. Similarly, in Table 59b of that publication, at page 219, in the longer term, until approximately the year 2014, a pin count for high performance integrated circuit packages of the order of 9,000 will be required. Again, as indicated in the table, there is no known solution for this type of package.
These aspects are the focus of the present invention.